In general, a rotor of an electric power generator or the like is manufactured by a method which comprises: refining molten steel in a steel-making furnace such as a converter, casting the molten steel into a bloom, hot-rolling the thus cast bloom into a steel bar, cold-forging the thus hot-rolled steel bar to prepare a rotor, and then, subjecting the thus prepared rotor to an annealing to impart same desired magnetic properties.
The above-mentioned annealing is applied to the rotor for the purpose of imparting desired magnetic properties including a high magnetic permeability and a low coercive force to the rotor. An annealing treatment however requires large-scale facilities and a considerable amount of thermal energy. If the annealing process can be omitted from the manufacturing processes of the rotor, therefore, it would permit simplification of equipment as well as saving of thermal energy.
As a method for manufacturing a steel sheet having excellent magnetic properties including a high magnetic permeability and a low coercive force by heating a slab as a material and hot-rolling the heated slab without applying the above-mentioned annealing, the following method has conventionally been proposed:
A method for manufacturing a hot-rolled high-tensile electrical steel sheet, as disclosed in Japanese Patent Provisional Publication No. 60-86,210 dated May 15, 1985, which comprises the steps of;
heating a slab consisting essentially of; PA0 carbon: from 0.06 to 0.09 wt. %, PA0 manganese: from 0.5 to 1.4 wt. %, PA0 silicon: up to 0.10 wt. %, PA0 aluminum: up to 0.10 wt. %, PA0 titanium: from 0.04 to 0.25 mt.%, and PA0 the balance being iron and incidental impurities, PA0 where, the respective contents of sulfur and nitrogen as said incidental impurities being: PA0 up to 0.02 wt. % for sulfur, and PA0 up to 0.01 wt. % for nitrogen, PA0 using a material consisting essentially of: PA0 carbon: from 0.02 to 0.08 wt. %, PA0 manganese: from 0.05 to 0.49 wt. %, and PA0 the balance being iron and incidental impurities, where, the respective contents of silicon, aluminum and nitrogen as said incidental impurities being: PA0 up to 0.10 wt. % for silicon, PA0 up to 0.02 wt. % for aluminum, and PA0 up to 0.004 wt. % for nitrogen; PA0 heating said material to a temperature of at least 1,000.degree. C.; then PA0 hot-working said material thus heated at a finishing temperature of at least 1,000.degree. C. to prepare a steel article; and then PA0 cooling said steel article thus prepared to a temperature of up to 500.degree. C. at a cooling rate of up to 0.5.degree. C./second; PA0 thereby causing crystal grains of said steel article to grow to a grain size of at least 50 .mu.m to impart a high magnetic permeability and a low coercive force to said steel article.
to a temperature of at least 1,200.degree. C.; then hot-rolling the thus heated slab into a steel sheet at a finishing temperature of at least Ar3 point and up to 900.degree. C.; and then coiling the thus hot-rolled steel sheet at a temperature of from 650.degree. to 500.degree. C. (hereinafter referred to as the "prior art").
The above-mentioned prior art involves the following problems: In the prior art, manganese is added to the steel sheet in order to improve the strength thereof. However, the manganese content of the steel sheet is as high as from 0.5 to 1.4 wt. %. This results in a deteriorated hot-workability and a low magnetic flux density in the steel sheet, leading to a lower magnetic permeability. In addition, in the prior art, titanium is added in an amount of 0.04 to 0.25 wt. % to the steel sheet in order to improve the strength thereof. As a result, a strain produced during hot-working tends to remain in the steel sheet, leading to a lower magnetic permeability. In the prior art, furthermore, the slab is hot-rolled into the steel sheet at a finishing temperature as low as up to 900.degree. C. in order to prevent the crystal grains of the steel sheet from coarsening. As a result, a strain produced during hot-working tends to remain in the steel sheet, leading to a lower magnetic permeability of the steel sheet.
Under such circumstances, there is a strong demand for the development of a method for manufacturing a steel article having, as compared with the above-mentioned prior art, more excellent magnetic properties including a maximum magnetic permeability .mu. max of at least 4,500 and a coercive force Hc of up to 1.2 Oersted (Oe), but such a method has not as yet been proposed.